It's always the zips that seem to break first on saddle packs where it has to go round tight corners ...so I'm using this ortleib one at the moment which doesn't have one and is great... Big enough for 2 tubes, multi tool and 2 or 3 CO2 cartridges and a couple of trye levers. http://www.ortlieb.co.uk/bike/saddle-bags/saddlebag-micro.html

I've got the medium Lezyne micro caddy and it fits snugly underneath my Selle San Marco Caymano saddle without any protrusion. The wide Velcro strap keeps it firmly in place over choppy terrain (cobblestone, rail tracks, potholed asphalt,...). Mine holds an inner tube, Crank Brothers 10-piece multi tool in the top pocket, tire levers in the side pocket, 2 CO2 canisters, and Lezyne CO2 inflator. I could also squeeze in some emergency cash too and an ID but I'd probably forget about it and lose it. Leaves plenty of space on my seat post for a rear light to be good and visible too.

The problem i have found with most saddle bags is that they aren't water proof. I went through several until I finally bought an Ortlieb http://www.ortlieb.de/_prod.php?lang=en&produkt=micro&biketoy=saddle It works great and tucks right under the seat. Everything inside stays nice and dry, which is a great plus point if you put your phone in there.

I've got a Micro Caddy Medium.
It's nice and small, just big enough for a tube, couple of gels and my keys. Has an internal money pocket, slot for levers and one for a patch kit. Also a separate pouch for a small multi tool.
For me it's the perfect size.

I have the micro caddy quick release. It's really great. And saves faffing when you need to remove it. Amazed at how great this is - reminds me what an awesome bike I have - with a saddle back it spoils the lines.

I got this for Christmas, so didn't really know that I would like it so much.

From looking at the dimensions on the Lezyne website, the medium is nearly twice as large in terms of capacity but whilst not being any longer or wider than the small one, just a bit deeper. I think that's definitely the one to get then if I'm trying to fit tubes and CO2 canisters in it.

Encouragingly, a couple of comments amongst the Wiggle reviews also suggest the Lezyne V10 multi-tool (being the one I've got) fits in the pocket underneath it.

I've been running one for 2 years and the construction, waterproofing and attachings are excellent. It has been known to swallow 2 tubes, a puncture kit, multi-tool, spoke key, screwdriver, energy bars, and my keys at one time. Also has a light fitting and pump elastic.

Looks good, but is waaaaay bigger than what I was after. The Mini version, at about half the size, is much more like it. Thanks for the recommendation.

Also stumbled across the Arundel Dual bag last night, which certainly looks different. There's definitely choice other than the usual Lezyne/Topeak suspects though, even if they do tick a lot of boxes.

Another medium Caddy user here, very happy with it. About as small as I'd want to go personally though. Any extra space when I've put my tube, tire levers, Park patches and a gel in I just fill up with an extra muesli bar or something.
I've got the quick-release version and I also really like the way it holds it a bit further back- I could occasionally feel other ones I've had on the back of my thighs which is annoying.

+1 for the Topeak Aero Wedge Large. I carry two tubes, tyre levers, small pump, multi tool and compact waterproof jacket. On long rides it expands to get much more in. Fits my Giant TCR Advanced without being wider than my saddle and doesn't catch my legs at all. I have seen smaller bags causing riding problems.